Multi-protocol or multi-command RFID system

ABSTRACT

A multi-protocol RFID interrogating system employs a synchronization technique (step-lock) for a backscatter RFID system that allows simultaneous operation of closely spaced interrogators. The multi-protocol RFID interrogating system can communicate with backscatter transponders having different output protocols and with active transponders including: Title 21 compliant RFID backscatter transponders; IT2000 RFID backscatter transponders that provide an extended mode capability beyond Title 21; EGO™ RFID backscatter transponders, SEGO™ RFID backscatter transponders; ATA, ISO, ANSI AAR compliant RFID backscatter transponders; and IAG compliant active technology transponders. The system implements a step-lock operation, whereby adjacent interrogators are synchronized to ensure that all downlinks operate within the same time frame and all uplinks operate within the same time frame, to eliminate downlink on uplink interference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to interrogatory systems. Moreparticularly, the present invention relates to an interrogatory systemhaving closely-spaced interrogators that simultaneously processdifferent tag protocols or commands.

2. Background of the Related Art

As discussed in U.S. Pat. No. 5,030,807 to Landt, RFID (radio frequencyidentification) systems use frequency separation and time domainmultiplexing in combination to allow multiple interrogators to operateclosely together within the bandwidth limitations imposed by radioregulatory authorities. In transportation and other applications, thereis a compelling need for interrogators to operate in close proximity. Inthe example of a toll collection system, many lanes of traffic areoperated side by side, and it becomes necessary to simultaneously readtags that are present in each lane. This introduces new challenges,particularly when a system is designed to communicate with tags ofdiffering protocols, requiring performance sacrifices.

Backscatter RFID systems, because they are frequency agile, can usefrequency separation to allow simultaneous operation of closely spacedinterrogators. However, the ability to operate with acceptableperformance is limited by the ability of the interrogator to rejectadjacent channel interference, and in the case where frequencies arere-used, co-channel interference. In addition, the interference impactof operating multiple interrogators in close proximity to one another iscomplicated by second and third order inter-modulation effects. Becausethe downlinks (interrogator to tag) are modulated signals and the uplinksignals (tag to interrogator) are continuous wave (CW) carriers at theinterrogator, the interference on an uplink by a downlink is more severein most cases than either downlink on downlink interference or uplink onuplink interference. When downlink on uplink interference debilitatesperformance beyond an acceptable level, the system could be set up fortime division multiplexing among the interrogators. Interrogators wouldthen share air time (take turns) according to a logic scheme to minimizeor eliminate the impact of the interference between interrogators. That,however, results in lower speed performance since a given transactionrequires more total time to complete. When a large number of lanes areinvolved, the speed performance loss can be severe and unacceptable.

Active RFID systems typically cannot use frequency separation due to thefact that cost-effective active transmitters operate on a fixedfrequency. These systems have therefore followed an approach ofoperating in a pure time division mode to prevent interference amongclosely located interrogators.

Downlink on downlink interference typically occurs when a tag receivesthe signals from two interrogators. If the interrogators are closelyspaced, the RF level of the two transmitted bit streams may becomparable. If significant RF from the adjacent interrogator is receivedduring bit period when none should be received, the tag may incorrectlydecode the message.

From a self-test perspective, RFID systems typically utilize what iscommonly known as a “check tag” to provide a level of confidenceregarding the health of the RFID system. The check tag can be anexternally powered device that responds only to a specific command orresponds only to its programmed identification number. It can be builtinto the system antenna or it can be mounted on or near the systemantenna. It can also be housed within the interrogator and coupled tothe system antenna via a check tag antenna mounted near the systemantenna. Though the check tag can take a variety of forms, onecommonality is that the check tag must be activated in some manner sothat the response can be read by the interrogator and remain inactiveduring normal operation.

When a check tag is activated, it typically provides a response that canbe read by the interrogating device. The check tag response is generallythe same as what would be received by the interrogator during normaloperation as a tag passes through the system in that particularapplication. If a backscatter RFID system initiates a check tag and aresponse is received, it verifies the RFID system is operational to thepoint that RF has been transmitted and the check tag backscatterresponse received and decoded. Encoded modulation of the RF is onlyverified if the check tag requires a modulated signal to trigger itsresponse. The time that it takes to complete the cycle depends upon thetype of tag utilized and can range from a few to several milliseconds,and the cycle is repeated periodically.

SUMMARY OF THE INVENTION

It is therefore one object of the present invention to provide aninterrogating system that is able to simultaneously operate a pluralityof closely-spaced interrogators. It is another object of the inventionto provide an interrogating system that synchronizes a plurality ofinterrogators. It is another object of the invention to provide a systemthat simultaneously processes different protocols used to communicatewith tags. It is another object of the invention to provide a systemthat simultaneously processes different backscatter protocols. It is yetanother object of the invention to provide a system that simultaneouslyprocesses different active and backscatter protocols. It is yet anotherobject of the invention to provide an interrogating system that avoidsinterference on an uplink by a downlink, as well as downlink on downlinkinterference, and uplink on uplink interference. It is yet anotherobject of the invention to provide a self-test operation that can verifyoperation of the interrogator and that does not have the timeconstraints of the check tag. It is another object of the invention toprovide an interrogation system in which uplink signals are received,and downlink signals are sent, over a single antenna.

In accordance with these and other objects of the invention, amulti-protocol RFID interrogating system is provided that employs asynchronization technique (step-lock) for a backscatter RFID system thatallows simultaneous operation of closely spaced interrogators. Theinterrogator can read both active and backscatter tags more efficientlywhen combined with time division multiplexing. The multi-protocol RFIDinterrogating system can communicate with backscatter transpondershaving different output protocols and with active transponders,including: Title 21 compliant RFID backscatter transponders; IT2000 RFIDbackscatter transponders that provide an extended mode capability beyondTitle 21; EGO™ RFID backscatter transponders, SEGO™ RFID backscattertransponders; ATA, ISO, ANSI AAR compliant RFID backscattertransponders; and IAG compliant active technology transponders.

The system implements a step-lock operation, whereby adjacentinterrogators are synchronized to ensure that all downlinks operatewithin the same time frame and all uplinks operate within the same timeframe. The step-lock operation allows for improved performance withhigher capacity of the RFID system. Active and backscatter technologiesare implemented so that a single interrogator can read tags of bothtechnology types with minimal interference and resulting goodperformance.

The step-lock operation eliminates downlink on uplink interference.Because downlink on uplink interference is the most severe form ofinterrogator-to-interrogator interference, that has the net impact ofreducing the re-use distance of a given frequency channel significantly.The step-lock technique can be extended to reduce or eliminate downlinkon downlink interference for fixed (repeating) downlink messages. Thiscan be achieved by having the interrogators transmit each bit in thedownlink message at precisely the same time. Depending on radioregulations and the number of resulting available frequency channelswith a given backscatter system, that can allow re-use distancessufficiently close that an unlimited number of toll lanes can beoperated without any need to time share among interrogators, drasticallyimproving performance and increasing capacity of the overall RFIDsystem.

Step-locking of the interrogators allows the interrogators to operate ina multi-protocol mode, whereby the same interrogator can read bothactive and backscatter tags in a more efficient way. This isaccomplished by combining a time division strategy for activetransponders and the step-locked frequency separation strategy forbackscatter tags into one unified protocol.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram of interrogators in a step-lock configurationwhere the synchronization signal is generated by the interrogator in aMaster/Slave mode;

FIG. 2 is a block diagram of interrogators in a step-lock configurationwhere the synchronization signal is generated by an external source;

FIG. 3( a) is a timing diagram of the step-lock feature showing theuplinks, downlinks, and processing times for multiple interrogators;

FIG. 3( b) is a timing diagram at the bit level;

FIG. 3( c) is a timing diagram of the step-lock feature having a timedivision multiplex;

FIG. 4 is a preferred block diagram of the interrogator;

FIG. 5 is a block diagram of the synthesized sources 33, 45 of FIG. 4;

FIG. 6 is a block diagram of the dual mixer configuration 56 of FIG. 4;

FIG. 7 is a block diagram of the DOM DAC and modulation control 60 ofFIG. 4;

FIG. 8 is a block diagram of the power amplifier 65 and its peripheralsof FIG. 4;

FIG. 9 is a block diagram of the downlink/uplink DACs and power control72 of FIG. 4;

FIG. 10 is a block diagram of the interrogator showing the loop-backbuilt-in-test capability;

FIG. 11 is a block diagram of the interrogator showing the test tagbuilt-in-test capability with a coupling antenna;

FIG. 12 is a block diagram of the interrogator showing the test tagbuilt-in-test capability with a directional coupler;

FIG. 13 is a lane plan for the system showing the downlink frequenciesfor a single protocol having different command sequences;

FIG. 14 is a lane plan for the system of FIG. 13, showing the uplinkfrequencies;

FIG. 15 is a timing chart for the system of FIGS. 13 and 14, showing thecommand sequences;

FIG. 16 is a lane plan for the system showing the downlink frequenciesfor active transponders and backscatter transponders;

FIG. 17 is a lane plan for the system of FIG. 16, showing the uplinkfrequencies;

FIG. 18 is a timing chart for the system of FIGS. 16 and 17, showing theprotocol sequences;

FIGS. 19 and 20 are lane plans for the system showing the downlink anduplink frequencies for active transponders and backscatter transponders;and,

FIG. 21 is a timing chart for the system of FIGS. 19 and 20, showing theprotocol sequences.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following detailed description of the preferred embodiment,reference is made to the accompanying drawings that form a part hereofand in which is shown by way of illustration a specific embodiment inwhich the invention may be practiced. This embodiment is described insufficient detail to enable those skilled in the art to practice theinvention, and it is to be understood that other embodiments may beutilized and that structural or logical changes may be made withoutdeparting from the scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined by the appendedclaims.

Turning to the drawings, FIG. 1 is a block diagram of the overall system10 in accordance with a preferred embodiment of the invention. Thesystem 10 depicts a single cluster of interrogators 12 and hosts orcontrollers 14 in a step-lock configuration, and various active orbackscatter transponders 11. As shown, the interrogators 12 communicatewith the transponders 11 in accordance with various tag protocols, TagProtocol 1 and Tag Protocol 2. The controller 14 controls and interfacesvarious system components, such as the associated interrogator 12,vehicle detection, and video enforcement, as may be required by thespecific application.

One interrogator 12 is designated as the master, while the rest of theinterrogators 12 are designated as slaves. The master interrogator 12generates a synchronization signal 16 and transmits it to the slaveinterrogators 12. The interrogators 12 are connected together via anRS-485 interface for multipoint communication in half-duplex operation,and the synchronization signal 16 is transmitted over that line. Theoverriding factors in master/slave designation are the timing parametersset in the respective interrogators 12 versus the reception of thesynchronizing signal 16. The timing parameters are set in eachinterrogator 12, such that the subsequent slave can become the master inthe event of a failure.

The interrogator 12 preferably has a single antenna 18 that is used totransmit the modulated downlink signal to interrogate a transponder 11.The single antenna 18 also transmits the CW uplink signal required toreceive the backscatter response of a backscatter transponder. Inaddition, the single antenna 18 receives the response from an activetransponder 11.

FIG. 2 is a block diagram of the system 20, showing interrogatorclusters 22 and associated hosts or controllers 24 in a step-lockconfiguration. An external source 26 is provided that generates thesynchronization signal 28. In the preferred embodiment, the externalsource 26 is a GPS receiver that has a 1 pps (pulse per second) signalthat is utilized to enable synchronization of the respective clusters22. The master interrogator locks a reference clock to the GPS 1 ppssignal, and uses the reference clock to generate the synchronizationsignal that is sent to the slave interrogators. The timing of the 1 ppssignal from a GPS unit is very precise, which allows each of theclusters to be synchronized together in time. This configuration isutilized when distance, or some other physical impediment, does notallow for a direct connection of the clusters 22. Generally, one GPSreceiver is required per cluster 22, and the interrogators 22 can thenbe connected as shown in FIG. 1 to synchronize the cluster to theexternal source.

FIG. 3( a) is a timing diagram showing several interrogators 10operating in step-lock. The diagram shows that all of the interrogators12 transmit their uplink and downlink signals at the same time. Wheninterrogators 10 are step-locked, the timing for each interrogator 10 iscontrolled so that the uplinks and downlinks all start and end at thesame time. That reduces interference caused by one interrogator'sdownlink signal interfering with another interrogator's uplink signal.By utilizing different frequency plans among the various tag protocols,the number of interrogators in a particular cluster can be increased.

As shown in FIGS. 1-2, the system polls a Title 21 backscattertransponder for specific information, and then polls an EGO backscattertransponder for specific information and the respective transpondersrespond accordingly. Each interrogator 12 transmits a Tag Protocol 1signal and Tag Protocol 2 signal to each of the transponders 11. TheTitle 21 backscatter tags 11 provide a backscatter response to thecorresponding Title 21 protocol signal, Tag Protocol 1, and the EGObackscatter tags 11 provide a backscatter response to the correspondingEGO protocol signal, Tag Protocol 2.

FIG. 3( a) shows the timing required to support two tag protocols. Asdepicted, the first tag protocol, Tag Protocol 1, has downlink anduplink periods that differ from the downlink and uplink durations of thesecond tag protocol, Tag Protocol 2. The tag protocols may also havedifferent processing times that follow the uplink of data. Thus, if thetag protocols are left unsynchronized, there is the strong potentialthat the downlink for either the first or second protocol of oneinterrogator would interfere with the uplink for either the first orsecond protocol of another interrogator. To avoid that interference, theinterrogators are step-locked so that the downlinks of the first tagprotocol end at the same time for all of the interrogators, and thedownlinks of the second tag protocol also end at the same time for allof the interrogators, as shown in the figure. The timing is controlledby a synch signal at the beginning of each cycle, which triggers thedownlink signal of Tag Protocol 1.

If only those two types of tags are being interrogated, then the signalpattern in FIG. 3( a) would repeat itself. If more tag protocols areused, then the uplink and downlink signals for the additional tags aretransmitted before the pattern is repeated. In some cases, a particulartag protocol may be transmitted multiple times before the interrogatorsswitch to a different protocol, such as if the tag needs to be readmultiple times or if the tag is read and then put to sleep by anadditional command.

Thus, the protocols are preferably implemented in a serial fashion,whereby each interrogator cycles through the various protocols beforerepeating the pattern and all the interrogators are processing the sameprotocol. That is, the downlink and uplink signals for Tag Protocol 1are processed by all of the interrogators at the same time, followed bya processing time and the downlink and uplink signals for Tag Protocol2. It should be apparent to one skilled in the art that the protocolsneed not be aligned in a serial fashion, but can be run simultaneouslyin a parallel fashion by synchronizing the downlink times across thedifferent protocols. That is, a first interrogator can process a firstprotocol downlink signal while a second interrogator processes a secondprotocol downlink signal. This type of step-lock is illustrated withrespect to commands of a single protocol, for instance, in FIG. 18,which is discussed below.

However, having the interrogators process the same protocols minimizesany delay between the various signals due to the different signalingdurations of the various protocols. For instance, if Interrogator 1processes Tag Protocol 1 and Interrogator 2 processes Tag Protocol 2, adelay would have to be introduced before the downlink of Tag Protocol 1since the downlink of Tag Protocol 2 is much longer, so that TagProtocol 1 is not uplinking while Tag Protocol 2 is still downlinking.As shown in FIG. 18, the time for each transmission is increased toallow for the longest command, which is the select or read command ofthe EGO protocol.

FIG. 3( b) is a diagram showing the step-lock technique extended to thebit synchronization level for the signals of FIG. 3( a). Eachinterrogator is step-locked and the transmission of each bit in thedownlink message is transmitted at precisely the same time. For bitsynchronization, the exact same command (bit for bit) has to betransmitted by each interrogator and is intended for protocols thatsatisfy that criteria.

FIG. 3( c) shows the timing using a time division multiplexing andstep-lock synchronization for an application that includes both activeand backscatter transponders. The synch signal initiates the signalcycle, which in this case starts with the first set of interrogators,Interrogators 1, 4, 7, generating a transmit pulse in accordance withTag Protocol 1, the active tag protocol.

The active protocol is sent in accordance with a time division multiplexscheme. The transmit pulses are offset to prevent interference thatcorrupts data received by the reader which might otherwise result fromclosely located tags. Accordingly, the active protocol is divided intothree time slots. In the first slot, the first interrogator and everythird interrogator transmit the downlink for the active tag protocol.Following the transmission of the downlink, every interrogator looks fora response from the tag. If an interrogator that transmitted thedownlink receives a response, that interrogator assumes that the tag isunder its antenna. If an interrogator that did not transmit the downlinkreceives a response, that interrogator assumes that the tag is under theantenna of a different interrogator. The interrogator will preferablyignore responses of tags that under the antenna of a differentinterrogator.

In the second and third time slots, the other interrogators transmit intheir respective slots, and each interrogator uses the same logic on thereceived signals to decide if a tag response is under their antenna.Following the completion of the active tag protocol, every interrogatortransmits the backscatter protocol downlink, and then looks for thebackscatter uplink signal from the tag.

Interrogators

The multiple protocols supported by the interrogator translate to thespecific requirements of the respective transponders. The tags can bepassive or active, battery or beam powered, with additional variablesthat are dictated by the physics of the transponder. Thus, theinterrogators 12 must be able to accommodate the different variables andrequirements for active and passive tags, as well as the differentcommands and backscatter protocols. In addition, the interrogators 12must be capable of adjusting itself to handle different protocol powerlevels, depths of modulation, duty cycle, speed (bit rates), frequencyof transmissions, receiver range adjustments, as well as tag andinterrogator sensitivity.

Since the interrogator controls the power of the signal reflected by abackscatter transponder, the uplink RF power level is utilized to setthe respective uplink capture zone for a backscatter transponder. Thedownlink RF power level is used to communicate with a transponder thatrequires a modulated command (Title 21, IT2000, EGO, SEGO backscattertransponders), or a trigger pulse (active transponder), before thedevice will respond. Thus, the RF downlink power is utilized toestablish a downlink capture zone for the transponders specified, and inthe case of backscatter transponders, can be different than the uplinkRF power level. In addition, the RF power level required by a beampowered transponder is much greater than that required by a batterypowered transponder. Closed loop control is implemented to maintaintight control of the dynamic RF power level that is required by thesystem.

The requirement to support multiple depth of modulation (DOM) levels isnecessary due to the fact that the transponder receiver dynamic range isdependent upon the DOM transmitted during the downlink. The base bandpath of the respective transponders can be AC or DC coupled where the DCcoupled path typically requires a larger modulation depth. Closed loopcontrol is implemented to maintain control of the dynamic DOM level fromprotocol to protocol.

The ability to adjust duty cycle provides the flexibility to compensatefor finite non-linearity in the interrogator modulation path and thecapability to optimize the duty cycle to the respective transponderrequirements. The duty cycle would typically be set at 50% with a smalltolerance, however, the ideal for a transponder type could be higher orlower. The adjustment of the duty cycle or pulse width aids in thetuning of the modulated signal to the transponder requirements and inthe derivation of transponder sensitivity to variations of duty cycle.

With the exception of the Title 21 and IT2000 protocols, the baud ratesare different for all the protocols. The ratio from the fastest protocolto the slowest protocol is in excess of 10-to-1. The interrogator mustaccommodate the different baud rates from the point of origin within theinterrogator through transmission while maintaining control of RF power,DOM and the emission mask. The frequency of transmission, and when toactually transmit, relates to the synchronization period and must bevariable in order to accommodate all combinations of protocols andcommand sequences.

Finite receiver adjustments provide the capability to vary thesensitivity level of the interrogator for each protocol. Ideally, thedefault would be to have the interrogator sensitivity level of eachprotocol approximately the same. In a multi-mode application thatrequires the sensitivity levels of respective protocols to be different,they can be adjusted accordingly. An example is a multiple protocolapplication with a beam powered transponder of one protocol and abattery powered transponder of another protocol. The capture zone of thebattery powered transponder can be adjusted to a certain degree by thelevel of RF transmitted. The same is true for the beam poweredtransponder, but to a much lesser degree. If it is desired to align thecapture zones, the receiver adjustment provides another degree offreedom. This adjustment is provided for the RF receive path and in theform of threshold levels in the base band receivers that must beexceeded for the signal to pass. This technique is also useful for theelimination of undesirable cross lane reads.

FIG. 4 is a preferred block diagram of the interrogator 12. Theinterrogator 12 has a transceiver 30, and a processor 100. Thetransceiver 30 provides the communications link to the transponder, andthe processor 100 provides the functional control of the interrogator10. The transceiver 30 is comprised of a transmitter chain thatgenerates the amplitude modulation (“AM”) and CW carriers, a receiver toaccept and process either the backscatter or active response of therespective transponder, and a controller to interface to the processorand provide the necessary control for the transmit and receivefunctions.

The transceiver 30 includes a transmitter chain and a receiver chain.The transmitter chain includes sources 33, 45, source select 44, MOD/CW56, RF AMP 65, filter 74, coupler 76, isolation 77 and coupler 78. Thereceiver chain includes filter 82, attenuator 84, select 86, receivers88, 92, baseband processor 94, and detectors 90, 96.

Transmitter

The transmitter chain begins with the generation of two synthesized RFsources, the downlink/uplink source 45 and the dedicated uplink source33. The sources 33, 45 are used to generate the uplink and downlinksignals, such as the ones shown in FIG. 3( a). A downlink/uplink source45 generates the first synthesized RF signal (S1), which is used as adownlink modulated source to interrogate, activate, and/or trigger atransponder. This source can also be used as an uplink continuous wave(CW) source to provide the communications link for the response of abackscatter tag. The uplink source 33 generates a synthesized RF source(S2), which is used as an uplink CW source to provide the communicationslink for the response of a backscatter tag. The sources 33, 45 aresynthesized low phase noise sources that aid in providing highbackscatter receiver performance with a single antenna.

Turning to FIG. 5, the sources 33, 45 include a frequency synthesizer34, loop filter 36, low phase noise voltage controlled oscillator (VCO)38, and a coupler 40. The coupler 40 has a gain block 39 to feedback theVCO 38 output back to the synthesizer 34 to comprise a low phase noisephase lock loop (PLL). The output of the PLL has a high isolation bufferamplifier to provide gain and isolate the PLL from the transmitterchain. The processor 100 initializes the S1 and S2 sources to fixedfrequencies through the controlling device 43 on the transceiver 30 viathe Clock, Data and Load signals. An adjustable oscillator (not shown)provides the reference signal for both the uplink synthesizer 33 and thedownlink/uplink synthesizer 45. The oscillator is adjustable to providethe capability to calibrate to an external standard reference.

Source selection circuitry 44, comprised of high isolation single-pole,single-throw (SPST) switches, is used for sources 33, 45 that feed intoa high isolation single-pole, double-throw (SPDT) non-reflective switch.That provides the ability to select either source 33 or 45, whilemaintaining a high degree of isolation between the sources 33, 45 tominimize the generation of inter-modulation products. The processor 100controls the state of the switches through the controlling device 43 onthe transceiver 30.

A local oscillator (LO) 48 for the direct conversion backscatterreceiver is coupled off of the output of the SPDT switch 45. It is fedinto a high isolation buffer amplifier (not shown) to provide gain andisolate the transmitter chain from the receiver-portion of thetransceiver 30. The LO level is fixed by a gain block, low-pass filteredand fed into a high isolation SPST switch (not shown) to provideadditional isolation from the active receiver. The processor 100controls the state of the SPDT switch of the source 45 through thecontrolling device 43 on the transceiver 30.

The MOD/CW block 56 provides the capability to modulate the respectivesource or place the source in a CW condition. As shown in FIG. 6, theMOD/CW block 56 is comprised of a dual mixer configuration separated bya gain block. That configuration provides a high dynamic range of linearAM modulation to aid in reducing the transmitted occupied bandwidth.Though this type of configuration can introduce non-linear second-ordereffects, utilizing the second mixer to provide the majority of the AMmodulation minimizes the distortion. The mixers 56 are driven at baseband with the respective protocols bit stream, trigger signal or DClevel, respectively, by amplifiers that provide the required drivelevels. The drive levels from the amplifiers produce the desired peaklevel for CW or the “high” and “low” condition when modulating.

Transmitter Bit Rate and DOM Adjustment

The difference between the respective data rates of the protocolsrequires a configuration that can support the data rates for all of theprotocols, while maintaining an emission mask that minimizes channelspacing in order to maximize the number of available channels. Bit rateadjustment is handled in the interrogator, FIG. 4, by the modulationcontrol block 60, which is shown in greater detail in FIG. 7. The DOMDAC & Modulation Control 60 utilizes a switch to select between thehigh-speed path and the low-speed path. The high-speed path accommodatesthe high-speed protocols, such as Title 21 and IT2000, and a low-speedpath accommodates the low-speed protocols, such as EGO, SEGO and atrigger pulse. The controlling device 43 on the transceiver 30 selectsthe desired path based on the protocol configuration indicated by theprocessor 100. Eighth-order low-pass filters provide the desiredemission mask for the supported protocols.

The control unit 60 receives a fixed DC reference level (VREF), whichsets the level that indicates the transmission of a “high” bit, or CWcondition as required, and is the same for all protocols. Adigital-to-analog converter (DAC) 70 sets the level that indicates thetransmission of a “low” bit, or the DOM (depth of modulation) level,which is retrieved from a memory in the controller 43 as required. TheModulation signal provides true logic control of an SPDT switch thatselects either the “high” condition or the “low” condition based on thestate of the Modulation signal.

Each protocol that requires a modulated downlink transmission from theinterrogator has a corresponding memory location in the controllingdevice 43 on the transceiver 30 that is calibrated to the DOM levelrequired for that protocol. Switching between the respective DOM levelsis handled by the controlling device 43 based on the protocolconfiguration indicated by the processor 100. The modulation controlunit 60 outputs a Filter Mod signal, which is used by the MOD/CW 56 tomodulate the signal in accordance with the desired protocol.

Transmitter Power Level Adjustment

The interrogator must also be able to accommodate the various powerlevels required by the various backscatter protocols and the activetransponder protocol. Power adjustment is handled in the interrogator,FIG. 4, by the RF AMP 65 and the power controller 72, which are shown ingreater detail in FIGS. 8 and 9. Turning to FIG. 8, the RF AMP 65 iscomprised of a gain block 64, voltage variable attenuator 66, RF switch,and a 900 MHz Integrated power amplifier 68. The gain block 64 providesthe desired level into the voltage variable attenuator 66. The voltagevariable attenuator 66 is utilized to vary the RF power based upon aVCTL Attn signal received from the power controller 72. The attenuator66 provides a fixed rise time when turning on RF power for CWtransmission and also to the DOM level prior to a modulatedtransmission.

The DL/UL DACs & Power Control 72 is shown in FIG. 9. A downlink DAC 71sets the RF peak power level required for a downlink transmission to atransponder. An uplink DAC 73 sets the RF power level required for anuplink transmission of CW for a response from a backscatter transponder.Selection between the low-pass filtered uplink and downlink levels ishandled by the Attn_Sel signal through an SPDT switch. Another SPDTswitch passes the selected DAC level or a preset reference level as theVCTL Attn signal, which is utilized to limit the dynamic range of thevoltage variable attenuator 66. Both the downlink and uplink powerlevels are calibrated independently to provide 15 dB of dynamic range in1 dB steps.

Each protocol requiring a downlink transmission from the interrogatorhas an independent memory location in the controlling device 43 to storethe static power level for the respective configuration. The same istrue for each protocol that requires an uplink transmission. Thecontroller 43 controls the sequence of the downlink and uplinktransmissions based on the protocol configuration and discrete inputsfrom the processor 100. The integrated power amplifier 68 is selected toprovide the maximum desired output at the RF port while maintaining ahigh degree of linearity. The RF switch is utilized to provide thenecessary OFF isolation when the active receiver is enabled.

Transmitter Signal Processing

A low-pass filter 74, coupler-isolator-coupler configuration 76, 77, 78completes the transmitter chain. The low-pass filter 74 attenuatesharmonic emissions. The first RF coupler 76 provides the feedbacknecessary for closed-loop control. The coupled signal from the coupler76 is fed into a 4-bit digital step attenuator 97 that provides 15 dB ofdynamic range in 1 dB steps. By providing the dynamic range in the powercontrol feedback path, the closed loop control of downlink and uplink RFoutput power is simplified and accuracy of the transmitted power levelis improved.

The 15 dB feedback attenuation range coincides with the 15 dB dynamicrange of the transmitter to set the respective power level for thedownlink or uplink transmission. The feedback attenuator is set suchthat the attenuation level set on the uplink or downlink transmission,plus the level set on the digital step attenuator 97 in the feedbackloop, always add up to 15 dB. That minimizes the dynamic range of thesignal after the digital step attenuator 97 to the highest DOM levelrequired by the supported protocols. The attenuator 97 output is fedinto a logarithmic RF power detector 98 that converts the RF signal intoa voltage equivalent that corresponds to the RF level detected.

In essence, the modulating signal is reconstructed at voltage levelsthat represent the peak value transmitted for a digital “high” on thedownlink, a digital “low” representing the DOM level, or the CW level onthe uplink. The voltage levels for a digital “high” and a CW conditionremain virtually the same for the entire 15 dB dynamic range fortransmit power due to the corresponding level set on the digitalattenuator in the feedback loop. The voltage level for a digital lowcorresponds to the respective DOM level set for the protocol beingtransmitted.

In normal operation, the signal representing the detected RF level isadjusted for temperature drifts seen by the detector circuit and scaledfor input into an analog-to-digital converter (ADC) 99. The output ofthe ADC 99 is fed into the controlling device 43 on the transceiver 30that provides control of peak power, CW power, and the DOM, by utilizingclosed loop algorithms. The isolator 77 provides isolation of thetransmitter from the Tx port and the antenna port. The final RF coupler78 provides the receive path from the antenna port to the Rx port.

Receiver

The receiver portion of the transceiver 30, FIG. 4, accepts andprocesses the backscatter and active responses of the respectivetransponders. The RF receive chain begins with a band pass filter 82that includes a pre-attenuator and a post-attenuator followed by a gainblock. The filter 82 establishes the pass band for the backscatterreceiver and encompasses the pre-selector for the active receiver aswell. The sensitivity attenuator 84 and gain block establishes the RFdynamic range of the receiver.

The sensitivity attenuator 84 is also adjustable based on the protocolselected, to provide the capability to independently adjust and tune thesensitivities of the respective protocols. The sensitivity attenuator 84is a 4-bit digital step attenuator that provides 15 dB of dynamic rangein 1 dB steps. This attenuator provides the capability to vary thesensitivity level of the interrogator for each protocol. From acalibration standpoint, the sensitivity level of each protocol would beset such that they are approximately the same provided they meetestablished limits. For instance, if the maximum sensitivity of oneprotocol is −66 dBm and the maximum sensitivity of another protocol is−63 dBm, both can be calibrated to −62 dBm assuming the limit is −60dBm. Adjusting for the active and backscatter receive sensitivities aidsin the alignment of the capture zone when operating in a multipleprotocol environment.

The select block 86 provides the capability to select between twodifferent receive paths, a backscatter receive path (along elements 92,94, 96) and an active receive path (along elements 88, 90), based on theprotocol selected. An RF switch is utilized to separate the backscatterreceive path and the active receive path. The processor 100 controls thestate of the switch through the controlling device 43 on the transceiver30.

The backscatter receive path includes the backscatter receiver 92,baseband processing 94, and zero crossing detectors 96. The backscatterreceiver 92 includes a 0 degree power divider, a 90 degree hybrid,isolators, and mixers. The 0 degree power divider allows for an I & Q(In-phase & Quadrature) configuration that has two signals, one that isin-phase and one that is 90 degrees out of phase. To produce the I & Qchannels, the LO 48 output is fed through the 90 degree hybrid. Thereceive and LO paths are then fed through isolators in their respectivepaths to provide the RF and LO inputs to mixers for direct conversion tobase band for processing by the baseband processing 94. The isolators inthe 0 degree path are required to isolate the active receiver from thetransmitter LO and provide a good voltage standing wave ratio (VSWR) tothe hybrid coupler, which results in good phase and amplitude balance.

The isolators in the 90-degree path are also required to provide a goodVSWR to the hybrid coupler. In the baseband processing 94, filter andamplifier paths are provided for high, medium, and low speed I & Qsignals to allow for the differing bandwidth requirements of therespective protocols. Zero-crossing detectors 96 convert the signalsinto a form required by the controlling device on the transceiver foradditional processing.

The active receive path includes an active receiver 88 and a thresholddetector 90. The active receiver 88 includes a band pass filter, gainblock and attenuation, logarithmic amplifier. The band pass filterestablishes the pass band and noise bandwidth for the active receiver.The gain block and attenuation combination establishes the dynamic rangeof the receiver in conjunction with a logarithmic amplifier thatconverts a received Amplitude Shift Keyed (ASK) transmission to baseband. The base band processing, which is part of the active receiver 88,does a peak detect and generates an automatic threshold to providegreater receiver dynamic range and signal level discrimination. A staticadjustable range adjust threshold sets the initial threshold level forthe threshold detector 90. The threshold level is selected so that thereceiver is not affected by noise by setting the initial threshold levelfor the threshold detector 90 above the receiver's noise floor level.The threshold level also aids in the alignment of the capture zone. In agiven application, the capture zone can be reduced from its maximum byincreasing this threshold level.

Dynamic Adjustments

The controlling device 43 on the transceiver 30 provides the necessaryfunctionality and control for factory calibration, initialization,source selection, DOM (closed-loop), RF power (closed-loop),transmitting and receiving, and built-in-test. The preferred embodimentof the controlling device 43 is a Field Programmable Gate Array and theassociated support circuitry required to provide the functionalitydescribed. The capability to factory calibrate is provided for thesynthesizer reference clock, depth of modulation, and RF power.Calibration of the reference clock is provided through a digitallycontrolled solid-state potentiometer that feeds into the voltagecontrolled frequency adjust port of the reference oscillator. Theoscillator is factory calibrated to a frequency standard that providesthe LO for the measuring device. The digitally controlled potentiometercontains on-board non-volatile memory to store the calibrated setting.

Depth of modulation calibration is provided for the levels required bythe supported protocols. The levels are 20 dB (IT2000), 30 dB (Title 21)and 35 dB (EGO, SEGO, IAG), which are stored in non-volatile memoryduring factory calibration. The respective levels are retrieved from thecontroller's 43 memory and loaded into the DOM DAC 70 based upon theprotocol that is selected and what the DOM level was set to for therespective protocol during the initialization of the transceiver 30.

RF power is calibrated in 1 dB steps over the 15 dB dynamic range forboth synthesized sources 33, 45. Each level is stored in non-volatilememory during factory calibration. The respective levels are retrievedfrom memory and loaded into the downlink and uplink attenuation DACs 72based upon the protocol that is selected and what the power level wasset to for the respective protocol during the initialization of thetransceiver 30.

The initialization process sets the frequency for the synthesizedsources S1, S2, as well as for the downlink attenuation, uplinkattenuation, source designation, duty cycle, base band range adjust andsensitivity adjust levels for the respective protocols. A clock, serialdata line, and a load signal are provided by the processor 100 to loadthe synthesizers 33, 45. A serial UART is used to pass attenuation,source designation, range and sensitivity adjust from the processor 100to the transceiver 30.

Source selection and transmit control is provided by the processor 100via configuration discretes that designate the selected protocol inconjunction with a discrete that indicates whether downlink or uplink isactive and a discrete for on/off control. Based upon the activeconfiguration and the parameters set during initialization, theappropriate attenuation levels are set from the calibrated values inmemory for the designated source. Acknowledge discretes are provided bythe transceiver 30 to facilitate sequencing. The sequence is dictated bythe respective protocol and is designed to maximize efficiency. Inaddition, an acknowledge message can be sent to the tag to activateaudio/visual responses as well as put the transponder to sleep for aperiod of time defined in the acknowledgement message. It is desirableto put a tag to sleep so that it doesn't continue to respond, such as ifthe vehicle is stuck in a lane, and so that the interrogator cancommunicate with other tags.

The RF power control for the downlink and uplink RF output power is aclosed loop system to provide stable power across frequency andtemperature, and stable DOM, independent of protocol. In accordance withthe preferred embodiment, the closed loop for DOM control includes thecontroller 43 (which includes the controlling algorithm), DOM controller60, MOD/CW 56, RF AMP 65, filter 74, coupler 76, attenuator 97, sensor98, ADC 99, and back to controller 43. The detected coupled output afterthe power amplifier provides the feedback path to the Field ProgrammableGate Array 43. The Field Programmable Gate Array 43 contains closed loopalgorithms for controlling both the CW uplink power levels and the peakpower levels for the modulated downlink. The closed loop power controlalgorithm samples the peak power level in the feedback path and comparesit to a factory calibrated power level reference. The control voltage(VCTL Attn) is adjusted through the DL/UL DAC & Power Control 72 to zeroout the error from the comparison.

The DOM control is also a closed loop system to provide stable DOMacross frequency and temperature, including for the RF AM DOM. Here, theclosed loop for the peak RF power control includes the controller 43(which includes the controlling algorithm), power controller 72, RF AMP65, filter 74, coupler 76, attenuator 97, sensor 98, ADC 99, and back tothe controller 43. The controller 43 includes a detected coupled outputafter a power amplifier that provides the feedback path to the FieldProgrammable Gate Array 43. The Field Programmable Gate Array 43contains closed loop algorithms for controlling the DOM for themodulated downlink. The closed loop DOM control algorithm samples theminimum power level in the feedback path and compares it to a factorycalibrated DOM reference for the respective protocol. The level withinthe Filter Mod signal that indicates the transmission of a “low” bit, orthe DOM (depth of modulation) level, will be adjusted through the DOMDAC & Modulation Control 60 to zero out the error from the comparison.

Receive control is provided by the processor 100 via configurationdiscretes that designate the selected protocol. The microprocessor 102generates the discretes, which in the preferred embodiment are fivesignals having a total of 32 unique modes. For instance, a discretesignal could be 00011, which signifies an EGO protocol and its specificparameters for operation. The discretes are sent to the controller 43,and the interrogator 12 configures itself to communicate with theselected tag by setting the appropriate power level, bit rates,backscatter path, and the like. Based upon the active configuration andthe parameters set during initialization, the appropriate receiver isactivated and the sensitivity adjust level is set from the calibratedvalues in memory for the respective protocol.

The processor 100 contains all of the necessary circuitry to perform orcontrol the various interrogator functions. It contains a microprocessor102 for running application code which controls manipulating and passingthe decoded tag data to the host, communications interfacing, interrupthandling, synchronization, I/O sensing, I/O control and transceivercontrol. The self test techniques (discussed below) for the systemutilizing the loop-back technique and the test tag technique are alsocontrolled by the processor 100 through the configuration controldiscretes.

Dynamic RF Power Adjustment

The ability to adjust the level of RF power transmitted serves multiplepurposes. Independent of transponder type and external interferingsignals, capture zones rely upon the RF power transmitted and the gainof the transmit/receive antenna. The multiple protocols supported by theinterrogator translates to the specific requirements of the respectivetransponders. They can be passive or active, battery or beam powered,with additional variables that are dictated by the physics of thetransponder. These variables include transponder receive sensitivity,turn on threshold, antenna cross section and conversion loss. To supportthese variables, the RF power of the interrogator must be adjustable tolevels stored in memory for each protocol such that the appropriatelevels are set when the respective protocol is selected.

Dynamic Depth of Modulation (DOM) Adjustment

The ability to select the DOM level of the transmitted downlink servesmajor purposes. Independent of transponder type and external interferingsignals, the transponders receiver dynamic range relies upon the DOMtransmitted during the downlink. The multiple protocols supported by theinterrogator translate to the specific requirements of the respectivetransponders. Their base band processing can be AC or DC coupled, withadditional variables that are dictated by the physics of thetransponder. To support these variables, the downlink DOM from theinterrogator must be selectable to levels stored in memory for eachprotocol such that the appropriate DOM is set when the respectiveprotocol is selected.

Dynamic Modulation Duty Cycle Adjustment

The ability to select the duty cycle for the base band downlinkmodulation provides the flexibility to compensate for finitenon-linearity in the modulation path and the capability to optimize theduty cycle to the respective transponder requirements.

A synchronous clock provides the capability to lengthen a “high” bit onthe modulated signal from the encoder to increase the duty cycle of thesignal provided to the DOM DAC & Modulation Control 60. Conversely,lengthening a “low” bit on the modulated signal from the encoderdecreases the duty cycle of the signal provided to the DOM DAC &Modulation Control 60. To support this capability, the duty cycle valueis retrieved from the memory of the controller 43 that was set duringthe initialization process for each protocol such that the appropriateduty cycle is set when the respective protocol is selected.

The independent adjustment of the duty cycle or pulse width aids in thetuning of the modulated signal to the transponder requirements and inthe derivation of transponder sensitivity to variations of duty cycle.For example, the Title 21 specification does not specify duty cycle orthe rise and fall times for the reader to transponder communicationprotocol. Consequently, manufacturers who build transponders that meetthe Title 21 specification produce transponders with characteristicsthat differ with respect to these parameters.

Dynamic Frequency Selection

Frequency selection is dynamic in the sense that there are separatedownlink and uplink sources 33, 45 that are fixed to specificfrequencies. In a typical single mode application with multipleinterrogators, the downlink (or modulated) frequency is set to the samefrequency on all of the interrogators and the uplink (or CW) frequencyis set to specific frequencies that are dependent on the respectiveprotocol. Higher data rate protocols require more separation betweenuplink frequencies but allow for frequency reuse across multiple lanes,i.e., use the same frequency in multiple lanes, without interference.Lower data rate protocols require less separation between uplinkfrequencies, however, frequency reuse becomes much more of an issue.

The interrogator 12 will typically operate on a single downlinkfrequency, so that only a single downlink synthesizer 45 is needed.However, the uplink signals can be sent on more than one frequency.Since each of the synthesizers 33, 45 operate at a fixed frequency, itwould be time consuming to switch the internal frequency for thatsynthesizer. Accordingly, two synthesizers can be used to send uplinksignals. The uplink synthesizer 33 can send an uplink signal on a firstfrequency, and the downlink/uplink synthesizer 45 can send an uplinksignal on a second frequency. It should be recognized, however, that theinvention can be implemented using more than one downlink frequency, andmore or fewer uplink frequencies.

Thus, when a high speed protocol and a low speed protocol are integratedinto a single multiple interrogator application, channel limitationsarise due to bandwidth limitations imposed by radio regulatoryauthorities. The system allows for this by the use of the step-lockarrangement and the capability to setup the interrogator to allow thedownlink source to be utilized as the uplink source for the low speedprotocol while the high speed protocol utilizes the dedicated uplinksource. This allows for the high speed and low speed protocols to bechannelized independently within the regulatory bandwidth limitationsand provides flexibility for the multiple protocol, multipleinterrogator application.

Self-Test Operation

The check tag system of the prior art is not well suited for use withthen multiple protocols of the present invention. The multiple checktags used to verify the respective signal paths place additional timeconstraints and inefficiencies on the system. Instead, turning to FIG.10, the system includes a self-test operation having the additionalcapability of synchronizing the self-test cycle within a cluster ofinterrogators 22. Backscatter operation requires that the interrogatortransmit uplink signals as a continuous wave (CW) in order to receivethe response from a backscatter transponder. Since the receiver isactive during the transmission of the uplink CW, it is possible for thebackscatter receivers to detect and process the downlink signal, whichis an amplitude modulated (AM) carrier.

The serial bit stream originating from the processor 100 via the encoder104 is looped back to the processor 100 via the decoder 106 as indicatedby the dotted lines. The loop starts at the encoder 104, and proceeds tothe controller 43 to the DOM DAC & Modulation Control 60, to the MOD/CW56, to the AMP 65, to the filter 74, to the coupler 76, to the isolation77, to the coupler 78, to the filter 82, to the sensitivity attenuator84, to the select 86. At the select 86, the Rx Select signal determinesthe path that the serial bit stream will take. One state will take itthrough the backscatter receiver 92 chain while the other state willtake it through the active receiver 88 chain.

As a result of the loop, the processor 100 is able to verify whether theserial bit stream through the decoder 106 matches the bit stream sentvia the encoder 104. If the serial bit stream sent by the encoder 104matches the bit stream received by the decoder 106, the microprocessor102 indicates that all of the elements along the test path are operatingproperly. However, even if the bit stream is off by a single digit, themicroprocessor 102 will indicate that the system is not operatingproperly. Preferably, the test bit stream is between 4 and 16 bits inlength, so that the test is fast, though a test could also have a bitstream length of an actual message, i.e., 256 bits.

Note that the active receiver 88 is tested as well with this process, ifit is active during the transmission of the downlink AM carrier, eventhough that is not the normal mode of operation and only viable from atest standpoint. The serial bit stream can be a simple pattern and veryshort in duration compared to the response from even the highest baudrate check tag. This method provides the means to confidence test thedownlink source, the RF transmitter chain, the active receiver and thebackscatter receivers. The uplink source can be tested in the samemanner by simply modulating what would normally be the CW source.

However, the loop shown in FIG. 10 does not provide a confidence test ofany components after the Tx/Rx coupler 78, i.e., the antenna, or the RFcable. To do so, the system uses the system shown in FIG. 11. The testtag 110 is a switching device connected to a coupling antenna that ismounted near the system antenna. The switching device is controlled bythe processor 100 to produce a backscatter response when coupled to theuplink CW transmitted from the system antenna. The serial bit stream forthe test tag 110 can be the same simple pattern utilized for theloop-back mode of FIG. 10, or it can be unique.

The system of FIG. 11 provides the means to confidence test the uplinksource, the RF transmitter chain, the backscatter receivers as well asthe antenna and coaxial cable. A full response can be simulated forbackscatter tags to facilitate more in-depth testing when it iswarranted. A simplified alternative to this method is shown in FIG. 12,where the transmitter is coupled directly to the test tag 110. Theself-test system can be used with any transmitter, receiver ortransceiver, and need not be used with a step-locking system or aninterrogator. In step-lock, the interrogator treats the test sequence asanother protocol so that the test occurs in the same time frame. Thus,in the embodiment of FIG. 3( a) for instance, the test sequence wouldoccur after the processing time of Tag Protocol 2 and prior to anotherSync Signal.

Illustrations

FIGS. 13-21 illustrate various embodiments of the system. In each ofthese embodiments, the system is designed to cover an unlimited numberof lanes, though preferably the system is used with up to about elevenlanes of traffic, plus four shoulder lanes. The system accommodates twoprimary protocols, the first protocol is for a tag sold under the tradename EGO. The first protocol has uplink frequencies that should not beshared since it could result in frequency instability. In addition,there must be at least 500 kHz clear spectrum around each uplinkchannel. The downlink channels can share the same frequency, or they canbe on different frequencies. The downlink spectrum from modulation willinterfere with uplink and must be kept out of the uplink receivebandwidth.

The second protocol is for an IT2000 tag. The second protocol has tagsthat wake up in three stages; RF power gets them to stage one, detectionof a downlink signal gets them to stage two, and stage three is the tagresponse to a read request. Uplink frequencies can be shared, andmultiple interrogators can use the same channel on the uplink. Theremust be at least +/−6 MHz of clear spectrum around each uplink channel.Downlink channels can share the same frequency, or they can be ondifferent frequencies. Downlink spectrum from modulation (either thefirst or second protocols) will interfere with the uplink signal andmust be kept out of uplink spectrum.

For the interrogators, the downlink and uplink frequencies cannot bechanged during operation, but remain fixed at their configurationfrequencies. All interrogators are step-locked to each other so thatthey are synchronous in time. The timing is controlled by the TDM signaland internal CAM files. Step-locking keeps the interrogators frominterfering with each other, and eliminates the need for shuttinginterrogators down during different time slots.

Single Tag Protocol

In the embodiment of FIGS. 13-18, a system is provided for tagsemploying a single signaling protocol, which is the IT2000 protocol inthis illustration. As best shown in the embodiment of FIG. 15, there areseveral different commands of different lengths that have to beexchanged between the interrogator 12 and the tag. Since the commandsare different lengths, the interrogator 12 adds dead time to the startof the shorter commands to ensure that all downlinks end at the sametime.

This mode utilizes a frequency plan with the downlink at 918.75 and theuplinks at 903 MHz and 912.25 MHz and 921.5 MHz. The downlink and uplinkare locked so that downlink signals do not interfere with uplinksignals. However, the interrogators do not have to be command locked.They are able to independently issue commands. That means that oneinterrogator may issue a read request while a interrogator in anotherlane is issuing a write request. Only the uplink and downlink aresynchronized. Since the downlinks happen at the same time, the uplinksdo not occur at the same time as the downlinks, thereby freeing up theentire spectrum for each of the uplink and downlink transmissions.

The downlink frequency plan is shown in FIG. 13. In this configuration,all downlinks are operating on the same frequency. FIG. 14 show theuplink frequency plan, where the uplinks use a three frequency reuseplan, namely 921.5 MHz, 912.25 MHz, and 903 MHz. As shown, the range foreach of the three different uplink frequencies do not overlap with oneanother, so that the frequencies are spaced across the lanes to reducethe interference between the interrogators. At the same time, eachfrequency is present in each of the three lanes, so that theinterrogator for each lane can receive information on any of the uplinkfrequencies. The oval patterns are created by positioning aninterrogator antenna 18 at the top of the oval.

In operation, upon power up or after a reset has occurred, theinterrogator is initialized with the parameters required for therespective application, such as the downlink and uplink frequencies.Protocol specific parameters are also set during initialization,including downlink and uplink power level, DOM level, sensitivityattenuation, range adjust, as well as source, receiver and transmitterassignments for the specific application protocol. Those parameterscorrespond to the five bit configuration assigned in the processor 100to the protocol.

Thus, for IT2000, a configuration of 00010 from the processor 100signals the transceiver 30 to retrieve the IT2000 specific parametersfrom the controller 43 memory for an impending communication sequence.The transceiver acknowledges the processor 100, and indicates that ithas received and set the appropriate parameters for the specificconfiguration. If it is a single protocol application, and theconfiguration does not change, occurs once since the transceiver 30 willthen be set to the appropriate configuration from that time forward.

The processor 30 turns on the transceiver 30 transmitter chain and anIT2000 command is encoded and transmitted on the downlink source at aspecific power and DOM level initialized for the IT2000 tags. Themodulation signal travels through the high-speed transmit filter pathset during initialization. Shortly after the downlink transmission iscomplete, the control signal changes state to turn the downlink sourceoff. This also turns the uplink CW source on at a specific power leveland enables the respective receive parameters that were set duringinitialization. If an IT2000 transponder response is received anddecoded through the high-speed backscatter path, it is processed at theend of the uplink CW transmission and the sequence repeats. All timingis tightly controlled to accommodate the step-lock techniques. Ifstep-lock is enabled, the sequences are keyed from the reception of thesynchronization signal.

Turning to FIG. 15, the timing of the various uplinks and downlinks isshown. The timing gives an overall time per slot of at least about 3.5ms, though the timing could be reduced to about just over 2 ms (the timeit takes to complete the longest transaction, if no processing time wasrequired. At 3.5 ms, the entire transaction takes a minimum of about 21ms. In 3.5 ms a vehicle travels 0.51 feet (100 mph), and in 21 ms avehicle travels 3.08 feet. Accordingly, the tag has the opportunity tocycle through the protocol several times prior to vehicle traveling adistance beyond the range required to uplink and downlink signals. For a10 foot read zone, the tag could complete approximately 3.3 entiretransactions.

As shown in FIG. 15, various downlink and uplink communication protocolsare utilized by the interrogator. The commands are defined in thefollowing Table 1. Thus, for instance, pursuant to the first command,Read Page 7, the interrogator sends a read request to the tag on thedownlink, and the tag sends a read response on the uplink.

TABLE 1 Protocol Commands Command Downlink Uplink Read Page 7 ReadRequest Read Response Read Page 9 Read Request with ID Read ResponseRandom # Request Random # Request Random # Response Write Page 9 WriteRequest with ID Write Response Write Page 10 Write Request with ID WriteResponse Gen Ack General Acknowledgement No Response

In the example of FIG. 15, a different interrogator 12 transmits each ofthe commands. Accordingly, the duration of the uplink, downlink, uplinkdead time, downlink dead time, and interrogator processing time differsfor each of the various commands. For instance, the Write Page 9 andWrite Page 10 commands have long downlink periods since information isbeing written. However, the signals are step-locked, so that all of thedownlinks end at the same time and the uplinks start at the same time.Thus, there is no interference between the uplink and downlinktransmissions.

Two Signaling Protocols

In the embodiment of FIGS. 16-18, a system is provided for tagsemploying two signaling protocols, which are the IT2000 and EGOprotocols in this illustration. FIGS. 16-17 show the spectrumrequirements for the frequency plan, with FIG. 16 showing the downlinkplan and FIG. 17 showing the uplink plan. The plan requires that thedownlink and uplink be synchronized for all interrogators. That meansthat during a certain time period all interrogators are transmittingtheir downlink signals. During the next time period the interrogatorsare transmitting their uplink signals. During these time periods theinterrogators may be supporting either of the two protocols. It is notrequired for the interrogators to be synchronized for the protocols,only that the downlink or uplink signals be synchronized.

During the downlink cycle, all of the interrogators transmit at 918.75MHz. During the uplink cycle, the odd IT2000 interrogators transmit at921.5 MHz, and the even interrogators transmit at 903 MHz. The EGOuplinks are spaced between 910 MHz and 915.5 MHz. The interrogators haveto be either IT2000 or EGO interrogators. The means that if lanecoverage requires 7 coverage areas, this implementation would require 14separate interrogators. Or if the interrogators are frequency agile,then the interrogator could switch between the required IT2000 uplinkfrequency and the required EGO uplink frequency depending on theprotocol being transmitted at that time.

Adding additional interrogators can cover additional lanes. The numberof EGO uplink channels that can be supported between 910 MHz and 915.5MHz limits the number of lanes. If the spacing between interrogators canbe reduced to 500 kHz, the number of EGO interrogators supported wouldbe 12. If additional EGO interrogators are needed then all the IT2000uplinks could be moved to 903 MHz and room for an additional 12 EGOinterrogators would be available between 915.5 MHz and 921.5 MHz. Thisconfiguration would support 24 EGO interrogators.

In operation, upon power up or after a reset has occurred, theinterrogator is initialized with the parameters required for therespective application, such as the downlink and uplink frequencies.Protocol specific parameters are also set during initialization,including downlink and uplink power level, DOM level, sensitivityattenuation, range adjust, as well as source, receiver and transmitterassignments for the specific application protocols. Those parameterscorrespond to the five bit configuration assigned to the respectiveprotocol.

A configuration of 00010 from the processor 100 signals the transceiver30 to retrieve the IT2000 parameters from memory for an impendingcommunication sequence. The transceiver acknowledges the processor 100,indicating that it has received and set the appropriate parameters forthe IT2000 protocol. The processor 30 then turns on the transceiver 30transmitter chain and an IT2000 command is encoded and transmitted onthe downlink source at a specific power and DOM level initialized forthe IT2000 protocol. The modulation signal travels through thehigh-speed transmit filter path set during initialization.

Shortly after the downlink transmission is complete, the control signalchanges states to turn the downlink source off. That also turns theuplink CW source on at a specific power level and enables the respectivereceive parameters that were initialized for the IT2000 protocol. If anIT2000 transponder response is received and decoded through thehigh-speed backscatter path, it is processed at the end of the uplink CWtransmission.

A configuration of 00011 from the processor 100 then signals thetransceiver 30 to retrieve the EGO parameters from memory for animpending communication sequence. The transceiver acknowledges theprocessor 100, thereby indicating it has received and set theappropriate parameters for the EGO protocol. The processor 30 turns onthe transceiver 30 transmitter chain and an EGO command is encoded andtransmitted on the downlink source at a specific power and DOM levelinitialized for the EGO protocol. The modulation signal travels throughthe low-speed transmit filter path set during initialization.

Shortly after the downlink transmission is complete, the control signalwill change states to turn the downlink source off. That also turns theuplink CW source on at a specific power level and enables the respectivereceive parameters that were initialized for the EGO protocol. If an EGOtransponder response is received and decoded through the low-speedbackscatter path, it is processed at the end of the uplink CWtransmission and the entire sequence will repeat. All timing is tightlycontrolled to accommodate the step-lock techniques. If step-lock isenabled, as in FIG. 3( a), the sequences are keyed from the reception ofthe synchronization signal. The IT2000 protocol is Tag Protocol 1 andthe EGO protocol is Tag Protocol 2.

FIG. 16 shows how the downlink frequency is used to cover a system thathas three lanes with coverage for the shoulders of each of the outsidelanes, and FIG. 17 shows the layout for the uplink frequencies. In thefigures, the circles represent the coverage achieved over an area of theroad surface. The numbers in the circles represent the individualinterrogators, with the number on the left for the IT2000 interrogatorand the number on the right for the EGO interrogator. The numbersassigned to each half-circle represent the frequency being used by thatparticular interrogator and matches up with a frequency on the left. TheIT2000 interrogators alternate between frequencies at 903 MHz and 921.5MHz. The IT2000 protocol allows the frequencies to be shared without theinterrogators significantly interfering with each other. The EGOinterrogators use the frequencies between 909.75 MHz and 915.75 MHz.Since each EGO interrogator requires a unique frequency for its uplink,the EGO frequencies are not shared.

FIG. 18 displays the timing required for the commands used by EGO andIT2000 tags. The first line is the EGO read command, which is a groupselect for the downlink and a work data (tag ID) on the uplink. This isthe only EGO command required for this illustration. Upon receiving thiscommand, the EGO tag reports back its ID. The rest of the commands arethe IT2000 commands listed in Table 1 above, which are completed in thesequence shown.

The critical timing location is the transition between the uplink anddownlink. That transition needs to occur at nearly the same time for allof the interrogators. If an interrogator stays in a downlink mode fortoo long, it could interfere with the uplink signals. The dead time forboth the uplink and downlink is the time that no commands are being sentor received by the interrogator. The interrogators generally use thedead time to align their downlink and uplink signals. The processingtime is the time required by the interrogator to process commandsreceived by the tag.

The interrogator alternates between an EGO Read command and an IT2000Read Page 7 command until it receives a tag response. An EGO tagresponse is processed during the uplink time and then is followed by anIT2000 Read Page 7 Command. The rest of the IT2000 commands follow anIT2000 tag response to the Read Page 7 Command.

By setting up the system the present way, an interrogator at one lanethat is processing an IT2000 tag does not force the rest of theinterrogators in the other lanes to wait until that tag is finished. Therest of the interrogators can continue to alternate between the IT2000and EGO reads. The system dramatically increases the time required toprocess an IT2000 command. The current IT2000 transaction takes around14 ms plus some additional transaction time. The minimum amount of timerequired for this process would be about 40 ms. If the interrogatormisses any commands and the missed commands have to be repeated, thetime would increase by about 7 ms per repeated command. At 100 mph, avehicle travels about 6 feet in 40 ms, which is a significant portion ofthe capture zone.

FIGS. 19-21 is another illustration of the system used with multiplebackscatter protocols, namely EGO and IT2000. In the presentillustration, the interrogators incorporate the capability of usingeither source 33, 45 as an LO in the receiver. This allows interrogatorsto use different frequencies for the EGO and IT2000 uplinks. Only onesource needs to be modulated since the EGO and IT2000 downlinks can beon the same frequency. All of the interrogators are step-locked in timeso that they are all performing the same operation at the same time.This ensures that no interrogators are transmitting while anotherinterrogator is trying to receiving.

In addition, a frame consists of an IT2000 command set and an EGOcommand set. During the IT2000 command set the entire IT2000 commandsequence is sent. Therefore, during one frame an IT2000 tag can be read,written to, and generally acknowledged off before the command setreturns to the EGO commands. The frame is approximately 14 ms induration covering both the EGO and IT2000 command set. In order toreduce the time required to complete the IT2000 transaction, the IT2000transaction has been reduced to a single read, single write and threegeneral acknowledgements.

FIG. 19 shows the spectrum requirements for the frequency plan. Theblocks represent the frequency location and bandwidth required for eachsignal. The IT2000 signals are wider because of IT2000's faster datarate requiring more spectrum. The figure shows that the EGO signals andthe IT2000 downlink signal share the same center frequency. Thesesignals use one of the sources in the interrogator while the othersource is used by the IT2000 uplink signals. The numbers in the blocksrepresent the different interrogators used to cover the lanes.

The IT2000 downlink and EGO uplink and downlink frequencies are spacedacross the 909.75 to 921.75 MHz band. The spacing requirement isdetermined by the selectivity of the EGO receive filters. The narrowerthe EGO uplink filters, the tighter the frequencies can be spaced andthe greater the number of lanes that can be supported. If the spacingcan be reduced to 500 kHz between channels, this setup supports 13interrogators. An additional two EGO interrogators could be added at 903and 921.5, by sharing the uplink signals used by the IT2000 channels.This would give a total of 15 interrogators, or the ability to support 6lanes and 4 shoulders.

FIG. 19 also shows a frequency plan for a 3-lane system for the IT2000downlink and the EGO interrogators. For this implementation, eachinterrogator is on a different frequency to eliminate the frequencyreuse issue associated with the EGO uplink. Lane discrimination isaccomplished by setting the correct power levels from the interrogators.To get more lane coverage the power is increased to reduce lane coveragethe power is decreased.

As shown in FIG. 20, the IT2000 uplink signals are at 903, 912.25, and921.5. The minimum spacing for IT2000 uplink is determined by theselectivity of the IT2000 receive filters. These filters need about 6MHzof spacing between channels. However, unlike the EGO uplink channels,the IT2000 uplink frequencies can be reused so that severalinterrogators can use the same channel.

FIG. 20 also shows the frequency plan for a 3-lane system for the IT2000uplink interrogators. For this implementation, the IT2000 uplinks sharethree center frequencies: 903, 912.25 and 921.5. Since the IT2000 uplinkchannels can reuse the same frequency, those frequencies are shared overseveral interrogators. The figure shows one method of setting up thelanes to reduce the co-channel interference by separating interrogatorsthat use the same frequency as far apart physically as can beaccomplished.

FIG. 21 shows the timing associated with step-locking all of theinterrogators together. For that system, all interrogators are lockedtogether on the same timing. Locking the signals together ensures thatno interrogator is performing downlink modulation while anotherinterrogator is attempting to receive an uplink signal. If that were tohappen, the downlink modulation could interfere with the uplink signaland block its reception.

The timing plan assumes that the IT2000 commands are reduced to a singleread, a single write, and three general acknowledgements (Gen Ack). Thesystem transmits the read request until it receives a read response andthen the rest of the read, write, and gen ack commands are completed. Inthis method, the system completes the entire read, write, and gen ackcommand set each cycle. The cycle time for these commands is around 14ms. At 100 mph a vehicle travels about 2 feet. If the read area is 10feet deep then the system should get between 4 and 5 reads depending onwhen in the cycle the tag enters the capture zone.

The foregoing description and drawings should be considered asillustrative only of the principles of the invention. The invention maybe configured in a variety of ways and is not intended to be limited bythe preferred embodiment. Numerous applications of the invention willreadily occur to those skilled in the art. Therefore, it is not desiredto limit the invention to the specific examples disclosed or the exactconstruction and operation shown and described. Rather, all suitablemodifications and equivalents may be resorted to, falling within thescope of the invention.

1. An interrogation system for communicating with a plurality oftransponders having different communication protocols, the interrogationsystem comprising a plurality of interrogators and a synchronizationsignal, each of the interrogators comprising a transmitter and areceiver the transmitters being arranged for transmitting downlinksignals in accordance with different communication protocols over adownlink communications link to the transponders, the receivers beingarranged for receiving respective uplink signals over an uplinkcommunications signal link from the transponders, the synchronizationsignal being arranged for synchronizing the downlink signals for each ofthe plurality of interrogators, each of the interrogators operating inresponse to the synchronization signal to simultaneously transmit thedownlink signals to the transponders to enable the transponders toprovide their respective uplink signals after the transmission of thedownlink signals, thereby enabling communication between respective onesof the interrogators and transponders without interference between theuplink signals from the transponders and the downlink signals from theinterrogators.
 2. The interrogation system of claim 1, wherein thetransponders comprise a backscatter transponder having a battery powersupply.
 3. The interrogation system of claim 1, wherein the transponderscomprise active transponders having active transmitters that transmit ata single fixed frequency.
 4. The interrogation system of claim 1,wherein the transponders comprise active transponders having activetransmitters that transmit at a single fixed frequency and backscattertransponders having battery power supplies.
 5. The interrogation systemof claim 1, wherein said plurality of interrogators comprise a firstinterrogator transmitting a first downlink signal in accordance with afirst communication protocol and a second interrogator transmitting asecond downlink signal in accordance with a second communicationprotocol.
 6. The interrogation system of claim 1, wherein saidsynchronization signal synchronizes the downlink signals and the uplinksignals so that the transmission of downlink signals do not interferewith the transmission of uplink signals.
 7. The interrogation system ofclaim 1, wherein said synchronization signal synchronizes the downlinksignals to end at substantially the same time.
 8. The interrogationsystem of claim 7, wherein said downlink signals are not all the samelength.
 9. The interrogation system of claim 1, wherein saidsynchronization signal synchronizes the downlink signals to start atsubstantially the same time.
 10. The interrogation system of claim 1,wherein said synchronization signal synchronizes the downlink signals tostart and transmit each bit at substantially the same time.
 11. Theinterrogation system of claim 1, wherein said synchronization signalsynchronizes the downlink signals so that the uplink signals start atsubstantially the same time.
 12. The interrogation system of claim 1,wherein the synchronization signal synchronizes the uplink signals foreach of the plurality of interrogators.
 13. The interrogation system ofclaim 1, said interrogator further having a processor providing atrigger signal to the transponder, polling the transponder for specificinformation, and providing an acknowledge message to the transponder inresponse to a valid response to the polling message being received. 14.The interrogation system of claim 1, wherein said downlink signalscomprise amplitude modulated radio frequency signals.
 15. Aninterrogation system for communicating with backscatter transpondersusing a communication protocol having different commands, theinterrogation system comprising; a plurality of interrogators, and asynchronization signal, each of the interrogators comprising atransmitter and a receiver, the transmitters being arranged fortransmitting downlink signals in accordance with different commands overa downlink communications link to the transponders, the receivers beingarranged for receiving respective uplink signals over an uplinkcommunications signal link from the transponders, the synchronizationsignal being arranged for synchronizing the downlink signals for each ofthe plurality of interrogators, each of the interrogators operating inresponse to the synchronization signal to simultaneously transmit thedownlink signals to the transponders to enable the transponders toprovide their respective uplink signals after the transmission of thedownlink signals, thereby enabling communication between respective onesof the interrogators and transponders without interference between theuplink signals from the transponders and the downlink signals from theinterrogators.
 16. The interrogation system of claim 15, wherein saidplurality of interrogators comprise a first interrogator transmitting afirst downlink signal in accordance with a first command and a secondinterrogator transmitting a second downlink signal in accordance with asecond command.
 17. The interrogation system of claim 15, wherein saidsynchronization signal synchronizes the downlink signals and the uplinksignals so that the transmission of downlink signals do not interferewith the transmission of up link signals.
 18. The interrogation systemof claim 15, wherein said synchronization signal synchronizes thedownlink signals to end at substantially the same time.
 19. Theinterrogation system of claim 18, wherein said downlink signals are notall the same length.
 20. The interrogation system of claim 15, whereinsaid synchronization signal synchronizes the downlink signals to startat substantially the same time.
 21. The interrogation system of claim15, wherein said synchronization signal synchronizes the downlinksignals so that the uplink signals start at substantially the same time.22. The interrogation system of claim 15, wherein the synchronizationsignal synchronizes the uplink signals for each of the plurality ofinterrogators.
 23. The interrogation system of claim 15, saidinterrogator further having a processor providing a trigger signal tothe transponder, polling the transponder for specific information, andproviding an acknowledge message to the transponder in response to avalid response to the polling message being received.
 24. Theinterrogation system of claim 15, wherein said downlink signals compriseamplitude modulated radio frequency signals.
 25. An interrogator capableof communicating with a first set of transponders and a second set oftransponders, the first and second set of transponders having differentpower, depth of modulation, or duty cycles, the interrogator comprising:a transmitter for transmitting a first downlink signal to the first setof transponders and a second downlink signal to the second set oftransponders; a receiver for receiving a first uplink signal from thefirst set of transponders and a second uplink signal from the second setof transponders; and, a controller for controlling said transmitter totransmit the first and second downlink signals based on the power, depthof modulation, or duty cycle of the respective first and second sets oftransponders, and for controlling said receiver to receive the first andsecond uplink signals based on the power, depth of modulation, or dutycycle of the respective first and second sets of transponders; whereinthe interrogator operates in an environment with at least one additionallike interrogator and all of the interrogators are responsive to asynchronization signal that enables the interrogators to simultaneouslytransmit downlink signals to the transponders to enable the transpondersto provide their respective uplink signals after the transmission of thedownlink signals, thereby enabling communication between respective onesof the interrogators and transponders without interference between theuplink signals from the transponders and the downlink signals from theinterrogators.
 26. The interrogator of claim 25, wherein said firstdownlink signal has a first power and said second downlink signal has asecond power.
 27. The interrogator of claim 25, wherein said firstdownlink signal has a first depth of modulation and said second downlinksignal has a second depth of modulation.
 28. The interrogator of claim25, wherein said first downlink signal has a first duty cycle and saidsecond downlink signal has a second duty cycle.
 29. The interrogator ofclaim 25, further comprising a voltage variable attenuator for adjustingthe power of said first and second downlink signals.
 30. Theinterrogator of claim 29, further comprising a power control unit havinga high speed filter, a low speed filter, and a switch responsive to saidcontroller for selecting said high speed filter or said low speed filterto provide a power level signal to said voltage variable attenuator toadjust the power of said first and second downlink signals.
 31. Theinterrogator of claim 30, wherein said voltage variable attenuator andsaid power control unit form a closed loop.
 32. The interrogator ofclaim 25, further comprising a modulation control unit having a highspeed filter, a low speed filter, and a switch responsive to saidcontroller for selecting said high speed filter or said low speed filterto adjust the depth of modulation of said first and second downlinksignals.
 33. The interrogator of claim 32, wherein said modulationcontrol unit forms a closed loop.
 34. The interrogator of claim 32,wherein the first set of transponders are active transponders and thesecond set of transponders are backscatter transponders.
 35. Theinterrogator of claim 32, wherein the first and second sets oftransponders are active transponders having active transmitters thattransmit at a single fixed frequency.
 36. The interrogator of claim 32,wherein the first and second sets of transponders are backscattertransponders.